The invention relates to electronic semiconductor devices, and, more particularly, to organic light emitting diode structures and fabrication methods.
Commercial light emitting diodes (LEDs) typically constitute a p-n junction of inorganic semiconductors, such as AlGaAs and GaAs; with the electron-hole recombination at the junction emitting a photon. In contrast, organic semiconductor LEDs (OLEDs) have various device structures with carrier recombination generating excitons (localized excited molecular states) which then lead to luminescence by the organic material. Rothberg and Lovinger, Status of and Prospects for Organic Electroluminescence, 11 J.Mat.Res. 3174 (1996) review current OLEDs and illustrate typical device structures in the form of a stack of thin layers with transverse current flow; the stack may be a transparent substrate (glass), a transparent anode (indium-tin oxide), a hole transport layer (TPD), an emissive layer which also is an electron transport layer and in which electron-hole recombination and luminescence occur (Alq3), and a cathode (Mg:Ag). TPD is N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1-biphenyl-4,4'-diamine and Alq3 is tris-8-hydroxyquinoline aluminum. The organic materials may be amorphous or polycrystalline discrete molecular or may be polymeric. Polymer layers differ from discrete molecular layers in that they are not deposited by vacuum vapor deposition, but rather by spin coating from an appropriate solvent. The polymeric layer may also be deposited (either by vapor deposition or spin coating) as prepolymer layers and then converted either thermally or photochemically to the active form.
FIG. 2b is a nominal energy level diagram for this device structure during operation; LUMO stands for lowest unoccupied molecular orbital and in a sense corresponds to a conduction band edge for an inorganic semiconductor, HOMO for highest occupied molecular orbital and corresponds to the valence band edge for an inorganic semiconductor, .PHI..sub.c the cathode work function, and V.sub.appl the applied voltage. In the usual case, the LUMO discontinuity from the electron transport layer to the hole transport layer is greater than the HOMO discontinuity in the opposite direction, so holes from the hole transport layer inject into the emitter layer and recombine with electrons to form excitons which excite the emitter layer to luminesce.
Huang et al, Enhanced Electron Injection in Organic Electroluminescence Devices Using an Al/LiF Electrode, 70 Appl.Phys.Lett. 152 (1997), report an OLED with a cathode of Al on a thin (0.5-1.0 nm) layer of LiF on the Alq3 emissive layer with a NPB hole transport layer plus a buffer of CuPc on the indium-tin oxide anode. NPB is N,N'-bis(1-napthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine and CuPc is copper phthalocyanine. The LiF barrier in the cathode seemingly provides band bending (LUMO level) in the Alq3 at the interface and thereby lowers the barrier height to electron injection into the Alq3 (the electrons tunnel through the LiF).
Dodabalapur et al, 30 Electron. Lett. 1000 (1994) describe a microcavity LED design to achieve full color and pixelation for OLEDs. Lithographic patterning of an inert filler layer is used in conjunction with a white organic emitter to derive red, green and blue light from a single electrical structure: electromagnetic boundary conditions enhance molecular emission of particular colors along the cavity normal which prescribes the color and leads to much higher efficiency than obtained with filters.
U.S. Pat. No. 5,656,508 shows integration of an array of OLEDs.